U.S. patent application number 14/084436 was filed with the patent office on 2015-05-21 for ceria-supported metal catalysts for the selective reduction of nox.
This patent application is currently assigned to Toyota Motor Engineering & Manufacturing North America, Inc.. The applicant listed for this patent is Toyota Motor Engineering & Manufacturing North America, Inc.. Invention is credited to Paul T. Fanson, Justin M. Notestein, Dario Prieto-Centurion, Charles Alexander Roberts.
Application Number | 20150139883 14/084436 |
Document ID | / |
Family ID | 52014398 |
Filed Date | 2015-05-21 |
United States Patent
Application |
20150139883 |
Kind Code |
A1 |
Notestein; Justin M. ; et
al. |
May 21, 2015 |
CERIA-SUPPORTED METAL CATALYSTS FOR THE SELECTIVE REDUCTION OF
NOX
Abstract
A composition and method for producing the same are provided.
The composition includes transition metal oxides adhered to a
surface of a cerium oxide support, and can additionally include
alkali metal or alkaline earth metal promotors. The method includes
incipient wetness impregnation of the support with metal salt in
solution, and can include impregnation with a metal chelator salt.
The composition can be useful as a catalyst for the reduction of
noxious gases in combustion exhaust streams. The composition can be
of particular use as a component of an automobile catalytic
converter, for the specific catalytic reduction of nitrogen oxides
to nitrogen gas.
Inventors: |
Notestein; Justin M.;
(Evanston, IL) ; Prieto-Centurion; Dario;
(Minneapolis, MN) ; Fanson; Paul T.; (Brighton,
MI) ; Roberts; Charles Alexander; (Ann Arbor,
MI) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Toyota Motor Engineering & Manufacturing North America,
Inc. |
Erlanger |
KY |
US |
|
|
Assignee: |
Toyota Motor Engineering &
Manufacturing North America, Inc.
Erlanger
KY
NORTHWESTERN UNIVERSITY
Evanston
IL
|
Family ID: |
52014398 |
Appl. No.: |
14/084436 |
Filed: |
November 19, 2013 |
Current U.S.
Class: |
423/263 |
Current CPC
Class: |
B01D 2255/202 20130101;
Y02T 10/12 20130101; B01D 2255/705 20130101; B01D 2255/2065
20130101; B01D 2251/202 20130101; B01D 2255/20746 20130101; B01D
53/945 20130101; B01D 2255/20738 20130101; B01D 2255/2022 20130101;
B01J 37/0205 20130101; B01D 2255/2025 20130101; B01J 37/0203
20130101; Y02A 50/20 20180101; B01D 2255/20761 20130101; B01J
37/0201 20130101; B01J 35/1019 20130101; Y02T 10/22 20130101; B01D
2255/20715 20130101; B01D 53/9413 20130101; B01J 37/0219 20130101;
B01J 23/78 20130101; B01J 23/83 20130101; B01J 23/70 20130101; B01D
2255/2027 20130101; B01J 23/10 20130101; B01J 37/024 20130101; Y02A
50/2324 20180101 |
Class at
Publication: |
423/263 |
International
Class: |
B01J 23/83 20060101
B01J023/83 |
Claims
1. A method for fabricating a catalyst comprising: contacting a
substrate containing ceria with a solution containing catalytic
cations; and contacting the substrate with a solution containing
promoter cations; wherein the catalytic cations consist of
transition metal cations, post-transition metal cations, or a
combination thereof, and the promoter cations consist of alkali or
alkaline earth metal cations or a combination thereof.
2. The method of claim 1 wherein the catalytic cations consist of
Period 4 transition metal cations.
3. The method of claim 1 wherein the catalytic cations consist of
iron, copper, or cobalt cations, or a combination thereof.
4. The method of claim 1 wherein contacting a substrate with a
solution containing catalytic cations is repeated at least one
time.
5. The method of claim 1 wherein the promoter cations consist of
alkali metal cations.
6. The method of claim 1 wherein the solution containing catalytic
cations comprises at least one chelator.
7. The method of claim 6 wherein the at least one chelator forms a
chelation complex with catalytic cation having a net negative
charge.
8. The method of claim 6 wherein the at least one chelator
comprises EDTA.
9. A catalyst comprising: a substrate that includes ceria;
catalytic cations bound to substrate surfaces; and promoter cations
bound to substrate surfaces; wherein the catalytic cations consist
of transition metal cations, post-transition metal cations, or a
combination thereof, and the promoter cations consist of alkali or
alkaline earth metal cations or a combination thereof.
10. The catalyst of claim 9 wherein the catalytic cations consist
of Period 4 transition metal cations.
11. The catalyst of claim 10 wherein the catalytic cations consist
of iron, copper, or cobalt cations, or a combination thereof.
12. The catalyst of claim 9 wherein at least 30% of catalytic
cations are able to undergo two or more reduction/oxidation cycles
at reduction temperatures not to exceed 550.degree. C.
13. The catalyst of claim 9 which catalyzes reduction of nitric
oxide.
14. The catalyst of claim 9 which catalyzes the reduction of nitric
oxide by carbon monoxide or propylene.
15. The catalyst of claim 9 which catalyzes the reduction of nitric
oxide by hydrogen gas.
16. The catalyst of claim 9 having N.sub.2 selectivity of greater
than about 67%.
17. The catalyst of claim 9 having N.sub.2 selectivity greater than
about 90%.
18. The catalyst of claim 9 wherein catalytic cations have a
loading density of .about.0.1-1.5% by weight, promoter cations
consist essentially of alkali metal cations, and promoter cations
are present in stoichiometric ratio to catalytic cations which
falls within a range of about 1:2 to 6:1, inclusive.
19. A catalyst fabricated by a method comprising: contacting a
substrate containing ceria with a solution containing catalytic
cations; and contacting the substrate with a solution containing
promoter cations; wherein the catalytic cations consist of
transition metal cations, post-transition metal cations, or a
combination thereof, and the promoter cations consist of alkali or
alkaline earth metal cations or a combination thereof.
20. The catalyst of claim 19 wherein the solution containing
catalytic cations comprises at least one chelator.
21. The catalyst of claim 20 wherein the at least one chelator and
catalytic cation form a chelation complex having a net negative
charge.
22. The catalyst of claim 20 wherein the chelator comprises
EDTA.
23. The catalyst of claim 19 wherein contacting the substrate with
a solution containing catalytic cations is repeated at least one
time.
24. The catalyst of claim 19 wherein catalytic cations consist of
Period 4 transition metal cations.
25. The catalyst of claim 24 wherein catalytic cations consist of
iron, copper, or cobalt cations or a combination thereof.
26. The catalyst of claim 19 wherein promoter cations consist of
alkali metal metal cations.
27. A catalytic converter comprising the catalyst of claim 19.
28. An automotive vehicle comprising the catalytic converter of
claim 27.
Description
TECHNICAL FIELD
[0001] The present invention relates in general to a composition of
matter including a support of cerium oxide modified with catalytic
cations and optionally with promoter cations, to a method for
fabricating the same, and in particular to the use of such a
composition in the specific catalytic reduction of nitrogen oxides
in automobile catalytic converter.
BACKGROUND
[0002] Automotive catalytic converter technology has evolved in
part with the replacement of two-way catalytic converter technology
by three-way catalytic converter technology. A three-way catalytic
converter receives its name from the fact that it is simultaneously
capable of catalyzing three chemical reactions involved in the
oxidoreductive removal of pollutants from a combustion exhaust
stream. These three reactions, in general form, are: i) oxidation
of unburned hydrocarbons to carbon dioxide, ii) oxidation of
incompletely burned carbon monoxide to carbon dioxide, and iii)
reduction of oxides of nitrogen (principally NO.sub.2 and NO) to
nitrogen gas (N.sub.2).
[0003] Three-way catalysts currently employ a support structure
such as cerium dioxide that is capable of storing oxygen for use
when oxygen effluent in the exhaust stream is low. The cerium
dioxide support can be doped with compounds such as oxides of
aluminum or zirconium to improve thermal stability, surface area,
and oxygen storage capacity. Catalytic centers which can be
composed of noble metal cations (platinum, palladium, or rhodium)
are incorporated into the support structure and can be direct
mediators of emission gas oxidation/reduction.
[0004] While current catalysts exhibit promising features in terms
of capacity, efficiency, and thermal stability, there is a constant
need to find further improvements in order to meet ever more
demanding regulatory requirements.
SUMMARY
[0005] A catalyst and a method for fabricating the catalyst are
disclosed herein. The catalyst discussed may have utility in
treatment of automotive emissions, or components of combustion
emission.
[0006] In one aspect, a method for fabricating a catalyst is
provided. The method comprises contacting a substrate containing
ceria with a solution containing catalytic cations; and contacting
the substrate with a solution containing promoter cations; wherein
the catalytic cations consist of transition metal cations,
post-transition metal cations, or a combination thereof, and the
promoter cations consist of alkali or alkaline earth metal cations
or a combination thereof. In some variations the solution
containing catalytic cations can comprise at least one chelator. In
some variations, the catalytic cations can consist of Period 4
transition metal cations. In some variations the promoter cations
can consist of alkali metal cations.
[0007] In another aspect, a catalyst is provided. The catalyst
comprises a substrate that includes ceria, catalytic cations bound
to substrate surfaces, and promoter cations bound to substrate
surfaces. The catalytic cations can consist of transition metal
cations, post-transition metal cations, or a combination thereof,
and the promoter cations can consist of alkali or alkaline earth
metal cations or a combination thereof. In some variations the
catalytic cations can consist of Period 4 transition metals. In
some variations the promoter cations consist of alkali metal
cations. In some instances the catalyst catalyzes Reaction I,
Reaction II, or both:
2NO+2H.sub.2.fwdarw.N.sub.2+H.sub.2O I,
2NO+3H.sub.2.fwdarw.2NH.sub.3+O.sub.2 II.
[0008] In some particular instances, the catalyst possesses %
N.sub.2 selectivity greater than about 67% or greater than about
90% according to Equation A:
% N 2 Selectivity = ( moles NO consumed by Reaction I moles NO
consumed by Reaction I and II ) .times. 100. A ##EQU00001##
[0009] In another aspect a catalyst and its method of fabrication
are provided. The method comprises contacting a substrate
containing ceria with a solution containing catalytic cations and
contacting the substrate with a solution containing promoter
cations, wherein the catalytic cations consist of transition metal
cations, post-transition metal cations, or a combination thereof,
and the promoter cations consist of alkali or alkaline earth metal
cations or a combination thereof. In some variations the solution
containing catalytic cations can comprise at least one chelator. In
some variations, the catalytic cations can consist of Period 4
transition metal cations. In some variations the promoter cations
can consist of alkali metal cations.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] Various aspects and advantages of the invention will become
apparent and more readily appreciated from the following
description of the various aspects taken in conjunction with the
accompanying drawings, of which:
[0011] FIG. 1 is a schematic representation of catalytic cations
bound in various configurations to a surface of a substrate
comprising ceria;
[0012] FIG. 2A is a flow-chart of one variation of a method for
fabricating a catalyst;
[0013] FIG. 2B is a flow-chart of another variation of a method for
fabricating a catalyst;
[0014] FIG. 3A is a kinetic trace of a nitric oxide reduction
reaction in the presence of a catalyst;
[0015] FIG. 3B is a kinetic trace of a nitric oxide reduction
reaction in the presence of a catalyst and including three periods
of deliberate water addition;
[0016] FIG. 4A is a DRUV-vis spectrum of a catalyst fabricated
using Fe(NO.sub.3).sub.3 on Na/CeO.sub.2;
[0017] FIG. 4B is a DRUV-vis spectrum of a catalyst fabricated
using NaFeEDTA on CeO.sub.2;
[0018] FIG. 5A is a temperature-programmed reduction trace of a
catalyst fabricated using Fe(NO.sub.3).sub.3 on Na/CeO.sub.2;
[0019] FIG. 5B is a temperature-programmed reduction trace of a
catalyst fabricated using NaFeEDTA on CeO.sub.2;
[0020] FIG. 6A is a deconvoluted temperature-programmed reduction
trace of a catalyst fabricated using NaFeEDTA on CeO.sub.2 at
moderate loading density;
[0021] FIG. 6A is a deconvoluted temperature-programmed reduction
trace of a catalyst fabricated using NaFeEDTA on CeO.sub.2 at high
loading density;
[0022] FIG. 7 is a graph of catalytic cation reduction for two
catalyst types at varying loading densities;
[0023] FIG. 8A is a redox cycled temperature-programmed reduction
trace of a catalyst fabricated using Fe(NO.sub.3).sub.3 on
Na/CeO.sub.2;
[0024] FIG. 8B is a redox cycled temperature-programmed reduction
trace of a catalyst fabricated using NaFeEDTA on CeO.sub.2;
[0025] FIG. 9A is a graph of first and second cycle catalytic
cation reductions of a catalyst fabricated using NaFeEDTA on
CeO.sub.2;
[0026] FIG. 9B is a plot of catalytic cation redox cycling fraction
as a function of loading density for a catalyst fabricated using
NaFeEDTA on CeO.sub.2;
[0027] FIG. 10A is XANES spectra of two reference iron oxides and a
catalyst fabricated using NaFeEDTA on CeO.sub.2 loaded at 1.5
Fenm.sup.-2 at varying temperature;
[0028] FIG. 10B is XANES spectra of two reference iron oxides and a
catalyst fabricated using NaFeEDTA on CeO.sub.2 loaded at 0.9
Fenm.sup.-2 at varying temperature;
[0029] FIG. 10C is XANES spectra of two reference iron oxides and a
catalyst fabricated using NaFeEDTA on CeO.sub.2 loaded at 0.6
Fenm.sup.-2 at varying temperature;
[0030] FIG. 10D is an XANES spectrum of two reference iron oxides
and a catalyst fabricated using Fe(NO.sub.3).sub.3 on Na/CeO.sub.2
loaded at 1.0 Fenm.sup.-2 at varying temperature;
[0031] FIG. 11 is plot of catalytic cation oxidation state as a
function of temperature and as measured by XANES for two types of
catalyst;
[0032] FIG. 12 is a bar graph of nitric oxide reduction rate as
catalyzed by a catalyst or by a promoter cation-modified substrate
at various temperatures;
[0033] FIG. 13A is a plot of nitric oxide reduction rate as a
function of loading density for two types of catalyst and for
promoter cation-modified substrate with a Pd-catalyst as
reference;
[0034] FIG. 13B is a plot of nitric oxide reduction turnover
frequency as a function of loading density for two types of
catalyst;
[0035] FIG. 14 is a plot of nitric oxide reduction reaction rate
for catalysts fabricated using NH.sub.4FeEDTA on CeO.sub.2 with
various alkali metal promoter cations;
[0036] FIG. 15 is a plot of nitric oxide reduction rate for a
catalyst fabricated using NH.sub.4FeEDTA on CeO.sub.2 with various
stoichiometric ratios of sodium promoter cations to catalytic
cations;
[0037] FIG. 16 is a bar graph of nitric oxide reduction rates for
various catalysts fabricated using protected cupric catalytic
cations;
[0038] FIG. 17 is a plot of nitric oxide reduction rates for
various catalysts fabricated using protected cupric catalytic
cations as a function of catalytic cation loading density; and
[0039] FIG. 18 is a bar graph of nitric oxide reduction rates at
varying temperature for three different catalysts fabricated with
iron, copper, or cobalt catalytic cations.
DETAILED DESCRIPTION
[0040] A catalyst comprising ceria substrate surface-bound by
catalytic cations, and a method for fabricating the catalyst, are
provided. The method for fabricating the catalyst can include
contacting a substrate with a solution containing catalytic cations
and with a solution containing promoter cations. In certain
variations, the solution containing catalytic cations can include
chelated catalytic cations. The catalyst can comprise a ceria-based
substrate, bound with catalytic cations of transition metal,
post-transition metal, or both and with promoter cations of alkali
metal, alkali earth metal, or both.
[0041] A method for fabricating a catalyst includes a step of
contacting a substrate with a solution containing catalytic
cations, alternatively referred to as a catalytic cation solution.
The substrate comprises ceria, where "ceria" refers to an oxide of
cerium. In different variations, the substrate can optionally
include various ceramic or metal oxides, for example oxide of
zirconium, aluminum, or silicon, in admixture with the ceria.
[0042] The substrate can include solid-phase material in any
physical configuration. In some aspects, the substrate can include
ceria which possesses high surface-area-to-mass ratio. In some
variations, a high surface-area-to-mass ratio can be greater than
about 20 m.sup.2g.sup.-1. In some variations, a high
surface-area-to-mass ratio can be greater than about 50
m.sup.2g.sup.-1. In some variations, a high surface-area-to-mass
ratio can be greater than about 75 m.sup.2g.sup.-1. In some
variations, a high surface-area-to-mass ratio can be greater than
about 100 m.sup.2g.sup.-1.
[0043] Suitable physical configurations of substrate that have high
surface-to-mass-ratio can include a honeycomb structure, any porous
structure, powder, or any other configuration which possesses high
surface-area-to-mass ratio. In some instances the substrate will be
present in a powder form with a particulate size of 25 nm to 200
.mu.M. In some examples, the substrate will be present in a powder
form with a surface-area-to-mass ratio of about 100
m.sup.2g.sup.-1.
[0044] Suitable catalytic cations employed in the method can be
taken from a group including transition metal cations and
post-transition metal cations. In some aspects, the catalytic
cations can include any transition metal cations or post-transition
metal cations. As used herein, a transition metal can be any
D-block metal of Groups 4-13. A post-transition metal can be any
metal from the group including aluminum, gallium, indium, tin,
thallium, lead, or bismuth. In some aspects, suitable catalytic
cations can include cations of one or more Period 4 transition
metals, a group including scandium, titanium, vanadium, chromium,
manganese, iron, cobalt, nickel, copper, and zinc. In some
instances, suitable catalytic cations will include cations of iron,
copper, or cobalt, or combinations thereof.
[0045] In some aspects, the solution containing catalytic cations
can include at least one chelator. As used herein, a chelator is a
soluble, multidentate ligand capable of forming more than one
coordinate bond with a central metal cation. Non-limiting examples
of chelators that can be employed include ethylene diamine
tetraacetic acid (EDTA), diethylene triamine pentaacetic acid
(DTPA), ethylene glycol tetraacetic acid (EGTA), nitrilotriacetic
acid (NTA), citric acid, ethylene diamine,
2,3-dimercapto-1-propanesulfonic acid, dimercaptosuccinic acid,
desferrioxamine mesylate, oxalic acid, tartaric acid, or ascorbic
acid. It is to be understood that a chelator can include either or
both of a conjugate acid/base pair.
[0046] It is contemplated that in variations of the method in which
a chelator is employed, a chelation complex between chelator and
catalytic cations can be pre-formed prior to inclusion of the
catalytic cations in the catalytic cation solution. As used herein,
a chelation complex can be a coordination entity comprising at
least one chelator molecule and a catalytic cation. In other
instances, chelator and catalytic cations can be introduced
separately to the solution containing catalytic cations.
[0047] Suitable chelators which can be employed in the method can
include any chelator which can form a thermodynamically stable
chelation complex with the catalytic cations. For example, a
chelator can be chosen which forms a chelation complex with
catalytic cations having a dissociation constant (K.sub.d) less
than 1 .mu.M under standard conditions (aqueous solution at neutral
pH, 25.degree. C., and 1 atm pressure). A chelator can be chosen
which forms a chelation complex having K.sub.d less than about 0.1
.mu.M under standard conditions. A chelator can be chosen which
forms a chelation complex having K.sub.d less than about 0.01 .mu.M
under standard conditions. A chelator can be chosen which forms a
chelation complex having K.sub.d less than about 0.001 .mu.M under
standard conditions. As used herein, the dissociation constant is
defined as the product of free catalytic cation concentration and
free chelator concentration divided by the sum of all chelation
complex species, including completely and partially coordinated
complexes.
[0048] In variations of the catalyst fabrication method which
employ a chelator, the catalytic cations can be described as
"protected catalytic cations". Without being bound to any
particular theory, and as illustrated schematically in FIG. 1, it
is believed that performance of the method can cause catalytic
cations to be bound to substrate surfaces in one or more distinct
configurations. These configurations include isolated ions or small
clusters, two-dimensional sheets, and three-dimensional
crystallites. Again, without being bound to any particular theory,
it is further believed that utilization of a chelator can influence
the distribution of catalytic cations on substrate surfaces, in
particular by inhibiting formation of isolated ions or small
clusters.
[0049] It is contemplated that in instances in which a chelator is
employed, chelator can be present in the catalytic cation solution
at nearly equimolar quantity relative to catalytic cations.
Alternatively, chelator can be present in molar excess of catalytic
cations. In some instances, a suitable chelator will be one capable
of forming a chelation complex with the catalytic metal cation, the
complex having a net negative charge. In some instances, a suitable
chelator will be a tetradentate ligand. In some instances, a
suitable chelator will be EDTA.
[0050] The catalytic cations of the catalytic cation solution can
comprise metal atoms in any positive oxidation state. Suitable
oxidation states can in some instances include those in which the
catalytic cations employed can form a thermodynamically stable
chelation complex with any chelator employed.
[0051] The catalyst fabrication method can additionally include a
step of contacting the substrate with a solution containing
promoter cations, alternatively referred to as a promoter cation
solution. Promoter cations can include cations of any element or
elements from a group consisting of alkali metals and alkaline
earth metals. In some instances, promoter cations can be cations of
at least one of the following elements: lithium, sodium, potassium,
rubidium, and cesium.
[0052] In some aspects of the method, the substrate can be
contacted with the solution containing catalytic cations, the
solution containing promoter cations, or both via incipient wetness
impregnation (IWI). In IWI, the substrate is wetted with the
incipient wetness volume, the minimum volume of solution needed to
fill intra- and inter-particle volume. The incipient wetness volume
can be determined by any suitable means. An example of suitable
means for determining incipient wetness volume of the substrate is
by total volume uptake during N.sub.2 physisorption.
[0053] The solution containing catalytic cations and the solution
containing promoter cations can each comprise a solvent. It is
contemplated that a suitable solvent will include any solvent
capable of solvating substrate surfaces, solvating promoter cation
or a salt thereof, solvating catalytic cation or a salt or
chelation complex thereof, or any combination of the
aforementioned. In some instances, the solvent can be water. In
some instances, the solvent can be mixed water/organic, such as
water/methanol. In some instances, the solvent can be a polar
organic solvent. The solution containing catalytic cations and the
solution containing promoter cations can comprise identical or
different solvents.
[0054] In some instances of the catalyst fabrication method that
include a step of contacting the substrate with a promoter cation
solution, the catalytic cation solution and the promoter cation
solution can be distinct. In different variations of such
instances, the substrate can be contacted with the promoter cation
solution before or during its contact with the catalytic cation
solution. In other instances the catalytic cation solution and the
promoter cation solution can be the same solution, i.e. a single
solution containing both promoter cations and catalytic cations can
be employed.
[0055] In instances where the substrate is contacted with a
promoter cation solution before the substrate is contacted with a
catalytic cation solution, the solvent of the promoter cation
solution can be removed from substrate prior to contacting the
substrate with the catalytic cation solution. Such solvent removal
can be achieved, for example, by heating or applying vacuum to the
substrate.
[0056] In some additional aspects, contacting the substrate with a
catalytic cation solution can be followed, directly or indirectly,
by removing the solvent of the catalytic cation solution. In other
additional aspects, contacting the substrate with a catalytic
cation solution can be followed, directly or indirectly, by heating
the catalyst under oxidative conditions. In yet other additional
aspects, contacting the substrate with a catalytic cation solution
can be followed, directly or indirectly, by solvent removal, and by
heating the catalyst under oxidative conditions. As used herein,
the phrase "under oxidative conditions" can mean: under conditions
sufficient to oxidize organic matter adsorbed to the catalyst. Such
organic matter could include, for example, chelator. Heating the
substrate under oxidative conditions can serve to burn off any
organic residues remaining from the IWI step(s) and or to evaporate
any volatile components retained in the composition. In some
variations the heating under oxidative conditions can comprise
heating the catalyst to about 550.degree. C. under ambient air.
[0057] In aspects of the method where contacting the substrate with
a catalytic cation solution is followed by solvent removal, a
sequence comprising those steps can be repeated at least one time.
Such repetition can be useful, for example, in instances where the
catalytic cation solution exists at concentration insufficiently
high to yield a desired loading density of catalytic cations bound
to the substrate.
[0058] FIG. 2A provides a schematic illustration of one possible
format of a method 100 for fabricating a catalyst according to the
present disclosure. In step 102, substrate can be modified by IWI
with a solution containing promoter cation, for example a solution
of NaHCO.sub.3. In step 104, the solvent is removed, for example by
placing the substrate under vacuum for an interval. Step 106
includes modification of the substrate by IWI with a solution
containing catalytic cations, for example a solution of
NH.sub.4FeEDTA. In step 108, the solvent is removed, for example by
placing the substrate under vacuum for an interval. In step 110,
the substrate is heated under oxidative conditions.
[0059] FIG. 2B illustrates another possible format of a method 112
for fabricating a catalyst according to the present disclosure. In
step 114, substrate is modified by IWI with a solution containing
promoter cations and catalytic cations, for example a solution of
NaFeEDTA. In step 116, the solvent is removed, for example by
placing the substrate under vacuum for an interval. In step 118,
the substrate is heated under oxidative conditions.
[0060] Also disclosed is a catalyst including a substrate and
catalytic cations bound to substrate surfaces. The substrate
comprises ceria, where "ceria" refers to an oxide of cerium. In
different variations, the substrate can optionally include various
ceramic or metal oxides, for example oxide of zirconium, aluminum,
or silicon, in admixture with the ceria. The catalytic cations can
be bound to substrate surfaces in geometries including isolated
ions or small clusters, two-dimensional sheets, or
three-dimensional crystallites, as schematically illustrated in
FIG. 1. In some variations, catalytic cations can be bound to
substrate surface predominantly in two-dimensional sheets.
[0061] A substrate can include solid-phase ceria in any physical
configuration. In some aspects, the substrate can include ceria
which possesses high surface-area-to-mass ratio. In some
variations, a high surface-area-to-mass ratio can be greater than
about 20 m.sup.2g.sup.-1. In some variations, a high
surface-area-to-mass ratio can be greater than about 50
m.sup.2g.sup.-1. In some variations, a high surface-area-to-mass
ratio can be greater than about 75 m.sup.2g.sup.-1. In some
variations, a high surface-area-to-mass ratio can be greater than
about 100 m.sup.2g.sup.-1.
[0062] It is to be understood that a substrate which possesses high
surface-to-mass-ratio can include substrate with a honeycomb
structure, substrate with a porous structure, a powder, or any
other configuration which possesses high surface-area-to-mass
ratio. In some instances the substrate will be present in a powder
form with a particulate size of 25 nm to 200 .mu.m. In some
examples, the substrate will be present in a powder form with a
surface-area-to-mass-ratio of about 100 m.sup.2g.sup.-1.
[0063] In various aspects, the catalytic cations, which are bound
to substrate surfaces, can comprise transition metal cations or
post-transition metal cations. As used herein, a transition metal
can be any D-block metal of Groups 4-13. A post-transition metal
can be any metal from the group including aluminum, gallium,
indium, tin, thallium, lead, or bismuth. Non-limiting examples of
suitable catalytic cations can include cations of cadmium, cobalt,
copper, chromium, iron, manganese, gold, silver, platinum,
titanium, nickel, niobium, molybdenum, rhodium, palladium,
scandium, vanadium, or zinc. In some aspects, suitable catalytic
cations can be those capable of forming a stable chelation complex
with a chelator.
[0064] In some aspects, the catalytic cations can be Period 4
transition metal cations, a group which includes cations of any of
the following: scandium, titanium, vanadium, manganese, iron,
cobalt, nickel, copper, or zinc. In some variations, the catalytic
cations can consist of cations of one element. In other variations,
the catalytic cations can include cations of more than one element.
It is contemplated that catalytic cations can be bound to substrate
surfaces by coordinate bonds. Such coordinate bonds can be oxide
bonds.
[0065] Catalytic cations can be present in the catalyst at any
loading density. As used herein, the phrase "loading density"
refers to the fraction of catalyst which consists of catalytic
cations. Loading density can sometimes be described in units of
weight percent, at other times in units of moles of catalytic
cations per mass of catalyst, and yet at other times in units of
number of catalytic cations per surface area of catalyst. In some
aspects, catalytic cations can be present at a loading density of
at least 0.1 wt %. In other aspects, catalytic cations can be
present at a loading density less than 2.0 wt %.
[0066] In some aspects, the catalytic cations can be present in the
catalyst in quantity sufficient to exceed monolayer coverage of
substrate surfaces. In other aspects, the catalytic cations can be
present in the catalyst in quantity sufficient to achieve but not
exceed monolayer coverage of substrate surfaces. In some aspects,
the catalytic cations can be present in the catalyst in quantity
insufficient to achieve monolayer coverage of substrate
surfaces.
[0067] The catalyst can additionally include promoter cations bound
to substrate surface. The promoter cations can include cations of
an alkali metal or an alkaline earth metal. In some instances,
suitable promoter cations can include cations of any element from a
group consisting of lithium, sodium, potassium, rubidium, and
cesium. It is to be understood that promoter cations can be bound
to substrate surfaces by coordinate bonds, including oxide bonds.
Oxides which can bind promoter cations to the substrate include
oxide of cerium and oxide of catalytic cation.
[0068] Promoter cations can be present in the catalyst in any
stoichiometric ratio relative to catalytic cations. In some
instances, the stoichiometric ratio of coordinated promoter cations
to catalytic cations can be within a range including 1:2, 6:1, or
any intermediate ratio.
[0069] In some aspects, the catalyst can catalyze the reaction of
gases which may be present in combustion exhaust, in particular
gases which may be present in the combustion exhaust stream of an
internal combustion engine. In some instances, the catalyst can
catalyze the reduction of an oxide of nitrogen. In some instances,
the catalyst can catalyze the specific reduction of nitric
oxide.
[0070] In various instances the catalyst can catalyze Reaction I,
Reaction II, or both:
2NO+2H.sub.2.fwdarw.N.sub.2+2H.sub.2O I.
2NO+3H.sub.2.fwdarw.2NH.sub.3+O.sub.2 II.
[0071] In instances where the catalyst catalyzes both Reactions I
and II, the catalyst can be characterized as having a percent
N.sub.2 selectivity as defined according to Equation A:
% N 2 Selectivity = ( moles NO consumed by Reaction I moles NO
consumed by Reaction I and II ) .times. 100. A ##EQU00002##
[0072] In some instances, the catalyst can catalyze Reactions I and
II with % N.sub.2 selectivity equal to or greater than about 67%.
In some instances N.sub.2 selectivity can be equal to or greater
than about 90%. FIG. 3A illustrates an instance of a catalyst
catalyzing Reactions I and II with N.sub.2 selectivity of about
67%. A catalyst was exposed to a 450.degree. C. gas flow containing
NO and H.sub.2, effluent was monitored by mass spectrometry for
N.sub.2 and NH.sub.3 content, and the results were converted to
turnover frequency (TOF, moles of product per mole of catalyst per
time) based on active catalytic cation quantity. Because the TOF is
proportional to NO consumption for each reaction, the results show
an approximately 2:1 preference for NO consumption by Reaction I,
or an approximately 67% N.sub.2 selectivity.
[0073] In other instances the catalyst can catalyze Reaction III or
Reaction IV:
2NO+2CO.fwdarw.N.sub.2+2CO.sub.2 III.
18NO+2C.sub.3H.sub.6.fwdarw.9N.sub.2+6H.sub.2O+6CO.sub.2 IV.
In such instances, the catalyst can additionally or alternatively
catalyze incomplete redox reactions between nitric oxide and carbon
monoxide or propylene. Incomplete reactions can have products such
as various nitrates or nitriles.
[0074] In certain aspects, the catalyst can be water stable,
retaining catalytic activity after exposure to water. Other than
through product formation, such exposure to water can occur, for
example, through the presence of water vapor in a combustion
exhaust stream to which the catalyst is exposed. Such an aspect is
illustrated in FIG. 3B. The experiment shown in FIG. 3B is similar
to that of FIG. 3A, but with the added feature of three temporary
additions of water vapor of one hour duration each indicated by
vertical dashed lines, wherein the H.sub.2 inlet stream was
diverted through an H.sub.2O bubbler. As shown, the catalyst
retains .about.90% activity during a water pulse and regains full
activity once the water pulse ends. The apparent spike in NH.sub.3
production during each water pulse is partially due to the mass
spectrometer's inability to distinguish water from ammonia.
[0075] In some aspects, at least 30% of the catalytic cations bound
to substrate can be capable of undergoing at least two
reduction/oxidation cycles under conditions where reduction
reaction temperatures do not exceed 550.degree. C. These aspects of
the catalyst are discussed in greater detail in Examples 41-44,
below.
[0076] Also disclosed is a catalyst comprising a substrate and
catalytic cations, fabricated by a method including contacting a
ceria substrate with a solution containing promoter cations and
contacting the substrate with a solution containing catalytic
cations. The substrate comprises a preparation of ceria, and can in
some variations include a ceramic or metallic oxide in admixture
with the ceria.
[0077] In various aspects, the substrate can include solid-phase
ceria in any physical configuration. In some aspects, the substrate
can include ceria which possesses high surface-area-to-mass ratio.
In some variations, a high surface-area-to-mass ratio can be
greater than about 20 m.sup.2g.sup.-1. In some variations, a high
surface-area-to-mass ratio can be greater than about 50
m.sup.2g.sup.-1. In some variations, a high surface-area-to-mass
ratio can be greater than about 75 m.sup.2g.sup.-1. In some
variations, a high surface-area-to-mass ratio can be greater than
about 100 m.sup.2g.sup.-1.
[0078] It is to be understood that a ceria support comprising ceria
which possesses high surface-to-mass-ratio can include ceria
substrate with a honeycomb structure, ceria substrate with a porous
structure, ceria powder, or any other configuration which possesses
high surface-area-to-mass ratio. In some instances the ceria
support will be present in a powder form with a particulate size of
25 nm to 200 .mu.m. In some examples, the ceria support will be
present in a powder form with a surface area of about 100
m.sup.2g.sup.-1.
[0079] Catalytic cations can include transition metal or
post-transition metal cations bound to substrate surfaces. In some
variations, catalytic cations can include Period 4 transition
metals. Catalytic cations can become coordinated by the surface of
the substrate when the substrate is contacted by a solution
containing catalytic cations. Catalytic cations can be coordinated
by the surface of the substrate via oxide bonds. Catalytic cations
can be arrayed on the surface of the substrate in isolated ions or
small clusters, two-dimensional sheets, three-dimensional sheets,
or combinations thereof.
[0080] A solution containing catalytic cations can include a salt
comprising catalytic cations. In some instances, a solution
containing catalytic cations can include a chelator. As noted
above, catalytic cations which are coordinated by a chelator can be
referred to as chelated catalytic cations, or as protected
catalytic cations.
[0081] Suitable promoter cations which can be employed in the
solution containing promoter cations can include cations of alkali
metal, cations of alkaline earth metal, or both. In some instances,
promoter cations can be cations of at least one element from a
group consisting of lithium, sodium, potassium, rubidium, and
cesium.
[0082] In some aspects, the catalyst can catalyze the reaction of
gases which may be present in combustion exhaust, in particular
gases which may be present in the combustion exhaust stream of an
internal combustion engine. In some instances, the catalyst can
catalyze the reduction of an oxide of nitrogen. In some instances,
the catalyst can catalyze the specific reduction of nitric
oxide.
[0083] As illustrated by the following Examples, catalysts
according to the disclosure which are fabricated by a method
wherein the solution containing catalytic cations includes chelated
catalytic cations can have catalytic properties different from
those of catalysts fabricated by a method in which chelated
catalytic cations are not employed. Without being bound to any
particular theory, it is believed that a method which employs
protected catalytic cations in the solution containing catalytic
cations can enhance the formation of two-dimensional sheets,
possibly by sterically and/or electronically blocking the catalytic
cations from adsorbing to isolated sites on substrate surfaces. In
this aspect, chelators which possess significant steric bulk and/or
chelators whose chelation complexes with catalytic cations have net
negative charge can be useful.
Examples
[0084] The following Examples are presented for illustrative
purposes only and are not to be interpreted as limiting the scope
of the present invention. The Examples will enable a clearer
understanding of the characteristics and advantages of the
invention.
[0085] Examples 1-33 describe the fabrication of various catalysts
according to the methods of FIGS. 3A, 3B. In Examples 1-19,
substrate was ceria and catalytic cations were ferric. Solutions
containing catalytic cations included either protected (NaFeEDTA or
NH.sub.4FeEDTA) or unprotected (Fe(NO.sub.3).sub.3) cations.
Examples whose fabrication included the use of NaFeEDTA were
fabricated according to the method of FIG. 3B. Examples whose
fabrication included the use of NH.sub.4FeEDTA or
Fe(NO.sub.3).sub.3 were fabricated according to the method of FIG.
3A, with steps 106 and 108 included in some, but not all, such
Examples.
[0086] Except as otherwise noted, substrate was NanoTek CeO.sub.2
powder, obtained from C.I. Kasei Co., Ltd, and having
surface-area-to-mass ratio of 101 m.sup.2g.sup.-1. Substrate was
dried at 120.degree. C. under ambient atmosphere for >12 h
before use. All other reagents were obtained from Sigma-Aldrich and
used as received. N.sub.2 physisorption isotherms were obtained
using a Micromeritics ASAP 2010 analyzer. Promoter-modified and
unmodified supports were degassed for 6 h at >5 mTorr and
120.degree. C. to release absorbed water prior to physisorption.
Elemental analysis of catalyst compositions was with a Varian MPX
ICP-OES instrument.
[0087] Examples 34-58 describe various analytical techniques
applied to the catalysts of Examples 1-33, probing their chemical,
structural, and catalytic properties.
Examples 1-3
[0088] NaFeEDTA and NH.sub.4FeEDTA were prepared by stifling 10
mmol H.sub.4EDTA and 10 mmol Fe(NO.sub.3).sub.3.9H2O in 25 mL of
H.sub.2O at 60.degree. C. until all solids dissolved. Once
dissolved, 40 mmol NaHCO.sub.3 or NH.sub.4 HCO.sub.3 was added
slowly, and the resulting solution was reduced to 5 mL by rotary
evaporation and stored at -20.degree. C. overnight. The resulting
crystals were separated by filtration, washed in acetone, and dried
at 20 mTorr for 12 hours. Thermogravimetric analysis indicated
>99% purity of the ferric chelates.
[0089] (NH.sub.4).sub.2CoEDTA was prepared by stifling 10 mmol
H.sub.4EDTA and 10 mmol Co(NO.sub.3).sub.3.6H.sub.2O in 25 mL of
H.sub.2O at 60.degree. C. until all solids dissolved. Once
dissolved, 40 mmol NH.sub.4HCO.sub.3 was added slowly, and the
resulting solution was completely reduced by rotary evaporation.
The complex was purified by heating under vacuum to remove residual
NH.sub.4NO.sub.3.
Examples 4-6
[0090] Catalysts were fabricated with a ceria substrate bound with
unprotected, ferric catalytic cations and sodium promoter cations
in a 1:1 stoichiometric ratio of catalytic-to-promoter cations.
Ceria powder was contacted with an IWI volume with concentration of
200, 400, or 600 mM NaHCO.sub.3. The modified substrates were dried
in a partially covered container under ambient conditions for 24
hours, then heated to 120.degree. C. for 12 hours. Each of the
modified substrates was then placed in an IWI volume of the same
concentration (200, 400, or 600 mM, respectively) of
Fe(NO.sub.3).sub.3. The catalysts were dried for 24 hours in
partially covered containers for 24 hours under ambient atmosphere,
and then for 12 hours under dynamic vacuum (.about.20 mTorr). The
dried catalysts were heated from room temperature to 550.degree. C.
under ambient, static air at a temperature ramp rate of 10.degree.
C./min.
Examples 7-13
[0091] Catalysts were fabricated with protected ferric catalytic
cations and sodium promoter cations in a single solution. CeO.sub.2
powder with surface area of 101 m.sup.2g.sup.-1 was placed in an
incipient wetness volume, determined by N.sub.2 physisorption, with
a concentration of 100, 200, or 300 mM NaFeEDTA as prepared in
Example 1. The impregnated materials were dried for 24 hours in
partially covered containers for 24 hours under ambient atmosphere,
and then for 12 hours under dynamic vacuum (.about.20 mTorr).
Because the solubility of NaFeEDTA limited the loading density from
a single impregnation to about 0.6 Fenm.sup.-2, multiples rounds of
impregnation and drying were used to achieve higher densities. The
dried catalysts were heated from room temperature to 550.degree. C.
under ambient, static air at a temperature ramp rate of 10.degree.
C./min.
Examples 14-15
[0092] Catalysts were fabricated with protected, ferric catalytic
cations and with no promoter cations. Ceria powder was placed in an
incipient wetness volume, determined by N.sub.2 physisorption,
containing 100 or 200 mM NH.sub.4FeEDTA as prepared in Example 2.
The impregnated materials were dried for 24 hours in partially
covered containers for 24 hours under ambient atmosphere, and then
for 12 hours under dynamic vacuum (.about.20 mTorr). The dried
catalysts were heated from room temperature to 550.degree. C. under
ambient, static air at a temperature ramp rate of 10.degree.
C./min.
Examples 16-19
[0093] Catalysts were fabricated with protected, ferric catalytic
cations and with a series of alkali metal promoter cations, at 1:1
stoichiometric ratios. Ceria powder was placed in an incipient
wetness volume of concentrations of 100 mM LiOH, NaHCO.sub.3,
KHCO.sub.3, or CsHCO.sub.3. The modified material was dried in a
partially covered container under ambient conditions for 24 hours,
then heated to 120.degree. C. for 12 hours to produce a series of
alkali-modified supports. The modified, dried material was then
placed in an IWI volume with a concentration of 100 mM
NH.sub.4FeEDTA as prepared in Example 2. The impregnated catalysts
were dried for 24 hours in partially covered containers for 24
hours under ambient atmosphere, and then for 12 hours under dynamic
vacuum (.about.20 mTorr). The dried catalysts were heated from room
temperature to 550.degree. C. under ambient, static air at a
temperature ramp rate of 10.degree. C./min.
Examples 20-22
[0094] Catalysts were fabricated with protected, ferric catalytic
cations at .about.0.5 Fenm.sup.-2 and with sodium promoter cations
at varying loading densities. Ceria powder was placed in an IWI
volume of concentration of 100, 200, or 300 mM NaHCO.sub.3 and the
modified substrates were dried in a partially covered container
under ambient conditions for 24 hours, then heated to 120.degree.
C. for 12 hours. The modified substrates were then placed in an IWI
volume of concentration 200 mM NH.sub.4FeEDTA as prepared in
Example 2. The impregnated materials were dried for 24 hours in
partially covered containers for 24 hours under ambient atmosphere,
and then for 12 hours under dynamic vacuum (.about.20 mTorr). The
dried catalysts were heated from room temperature to 550.degree. C.
under ambient, static air at a temperature ramp rate of 10.degree.
C./min.
Examples 23-30
[0095] Catalysts were fabricated with protected cupric catalytic
cations and sodium promoter cations in a single solution. Ceria
powder with surface area of 101 m.sup.2g.sup.-1 was placed in an
incipient wetness volume, determined by N.sub.2 physisorption,
containing 100, 200, or 300 mM Na.sub.2CuEDTA purchased from Sigma.
The impregnated materials were dried for 24 hours in partially
covered containers for 24 hours under ambient atmosphere, and then
for 12 hours under dynamic vacuum (.about.20 mTorr). Because the
solubility of Na.sub.2CuEDTA limited the loading density from a
single impregnation to about 0.6 Cunm.sup.-2, multiple rounds of
impregnation and drying were used to achieve higher densities. The
dried catalysts were heated from room temperature to 550.degree. C.
under ambient, static air at a temperature ramp rate of 10.degree.
C./min.
Examples 31-33
[0096] CeO.sub.2/ZrO.sub.2 powder with surface area of 108
m.sup.2g.sup.-1 was placed in an IWI volume of KHCO.sub.3. The
impregnated material was dried in a partially covered container
under ambient conditions for 24 hours, then heated to 120.degree.
C. for 12 hours to produce the alkali-modified substrate. The
promoter cation modified substrate was placed in an incipient
wetness volume with concentration of 200 mM (NH.sub.4).sub.2CoEDTA.
The impregnated material, in partially covered containers, was
dried for 24 hours under ambient atmosphere, and then for 12 hours
under dynamic vacuum (.about.20 mTorr). Otherwise equivalent
catalysts were fabricated using protected ferric or cupric
catalytic cations rather than cobaltous. Stoichiometric ratios of
promoter cations to catalytic cations were 1:1 for the ferric and
cobaltous containing catalysts, and 2:1 for the cupric containing
catalyst. The loading density of catalytic cations was .about.0.4
cations per nm.sup.2, equivalent to about 0.4% by weight.
TABLE-US-00001 TABLE 1 Characteristics of the materials discussed
in Examples 4-31 Catalytic Cation Promoter Cation Catalytic Cation
Loading Density Cat./Prom. Example Precursor Precursor wt % .mu.mol
g.sup.-1 atom nm.sup.-2 Ratio 4 Fe(NO.sub.3).sub.3 NaHCO.sub.3 0.43
82 0.49 1:1 5 Fe(NO.sub.3).sub.3 NaHCO.sub.3 0.87 164 0.98 1:1 6
Fe(NO.sub.3).sub.3 NaHCO.sub.3 1.35 255 1.52 1:1 7 NaFeEDTA N.A.
0.18 32 0.19 1:1 8 NaFeEDTA N.A. 0.34 60 0.36 1:1 9 NaFeEDTA N.A.
0.52 92 0.55 1:1 10 NaFeEDTA N.A. 0.53 94 0.56 1:1 11 NaFeEDTA N.A.
0.81 145 0.87 1:1 12 NaFeEDTA N.A. 1.16 207 1.23 1:1 13 NaFeEDTA
N.A. 1.38 246 1.47 1:1 14 NH.sub.4FeEDTA none 0.23 41 0.24 1:0 15
NH.sub.4FeEDTA none 0.48 86 0.51 1:0 16 NH.sub.4FeEDTA LiOH 0.22 40
0.27 1:1 17 NH.sub.4FeEDTA NaHCO.sub.3 0.23 41 0.27 1:1 18
NH.sub.4FeEDTA KHCO.sub.3 0.23 42 0.28 1:1 19 NH.sub.4FeEDTA
CsHCO.sub.3 0.22 39 0.26 1:1 20 NH.sub.4FeEDTA NaHCO.sub.3 0.47 83
0.50 2:1 21 NH.sub.4FeEDTA NaHCO.sub.3 0.49 87 0.52 1:1 22
NH.sub.4FeEDTA NaHCO.sub.3 0.49 87 0.52 2:3 23 Na.sub.2CuEDTA N.A.
0.23 36 0.22 1:2 24 Na.sub.2CuEDTA N.A. 0.31 49 0.29 1:2 25
Na.sub.2CuEDTA N.A. 0.36 57 0.34 1:2 26 Na.sub.2CuEDTA N.A. 0.61 96
0.57 1:2 27 Na.sub.2CuEDTA N.A. 0.54 86 0.51 1:2 28 Na.sub.2CuEDTA
N.A. 0.89 139 0.83 1:2 29 Na.sub.2CuEDTA N.A. 1.21 191 1.14 1:2 30
Na.sub.2CuEDTA N.A. 1.51 236 1.41 1:2 31 NH.sub.4CoEDTA KHCO.sub.3
0.42 71 0.4 1:1
Examples 34-35
Diffuse Reflectance UV-Vis
[0097] Catalysts fabricated according to Examples 4-13 were
analyzed by diffuse reflectance UV-visible (DRUV-vis) in order to
evaluate the geometry of coordination of catalytic cations on
substrate surfaces, as a function of catalytic cation loading
density and protected or unprotected nature of the catalytic
cations. Isolated Fe.sup.3+ ions are expected to absorb below 300
nm, two-dimensional sheets are expected to absorb between 300 nm
and 500 nm, and three-dimensional crystallites are expected to
absorb above 500 nm. Bands corresponding to isolated Fe.sup.3+ are
obscured by the dominant absorption of CeO.sub.2 below 350 nm,
while bands corresponding to two-dimensional sheets and
three-dimensional crystallites can appear as a shoulder on the
CeO.sub.2 band and as a peak centered at .lamda.>500 nm,
respectively.
[0098] DRUV-vis experiments were performed on a Shimadzu 3600
UV-visible-NIR spectrometer with a Harrick Praying Mantis diffuse
reflection attachment and with PTFE as the baseline standard. All
materials were diluted 1:5 in PTFE and ground with a mortar and
pestle before collecting spectra. Diffuse reflectance spectra were
converted to pseudo-absorption spectra using the Kubelka Munk
transform (Kubelka et al. Z. Tech. Phys. 12, 593-6011 (1931)).
[0099] Representative DRUV-vis spectra are shown in FIGS. 4A and 4B
for catalysts incorporated with sodium promoter cations and with
varying loading densities of ferric catalytic cations, where the
light lines are the catalysts and the heavy lines are ceria
substrate as reference. The catalysts shown in FIG. 4A were
fabricated according to Examples 4-6, while the catalysts shown in
FIG. 4B were fabricated according to Examples 7-13. The spectra of
FIG. 4A have minimal shoulder on the ceria peak below 500 nm and
virtually none above 500 nm, even at the highest ferric loading
densities. This suggests that unprotected catalytic cations are
incorporated in the catalyst predominantly as isolated ions or
small clusters.
[0100] The spectra of FIG. 4B have a shoulder below 500 nm even at
the lowest catalytic cation loading densities. At the highest
loading densities, particularly above 0.6 catalytic cations per
nm.sup.2, the spectra in FIG. 4B have large shoulders below 500 nm,
and significant absorption above 500 nm. This suggests that use of
protected catalytic cations in the catalyst fabrication method
enhances incorporation of catalytic cations into two-dimensional
sheets and, at high loading densities, into three-dimensional
crystallites.
Examples 36-40
Temperature Programmed Reduction
[0101] Temperature-programmed reduction (TPR) experiments were
performed on catalysts fabricated according to Examples 4-13 in
order to evaluate the catalytic cation reduction (H.sub.2O release)
for catalysts fabricated with protected vs. unprotected catalytic
cations. Monomeric catalytic cation structures interacting strongly
with substrate oxide support are expected to reduce at higher
temperature than agglomerates more closely resembling bulk oxides
of catalytic cations. For example, isolated ferric cations are
expected to reduce between 600.degree. C. and 700.degree. C., while
two-dimensional sheets and three-dimensional crystallites should
reduce at around 400-500.degree. C. and 300-400.degree. C.,
respectively.
[0102] TPR experiments were performed on a TA Instruments Q500 TGA
equipped with a Pfeiffer Thermostar Q200 process mass spectrometer.
The compositions were gradually heated to 550.degree. C. at a
temperature gradient of 10.degree. C.min.sup.-1 under a flow of 90%
O.sub.2/10% He at 100 standard cubic centimeters per minute (sccm)
and held for 15 minutes. The heat treated samples were cooled in
100 sccm He to near ambient temperature before being heated at
10.degree. C.min.sup.-1 to 550.degree. C. in 100 sccm 4.5% H.sub.2,
4.5% Ar, 91% He. The evolved water signal was normalized against
the constant argon signal and calibrated against a cupric oxide
reference standard.
[0103] Representative TPR scans can be seen in FIGS. 5A, 5B. Those
in FIG. 5A are of catalysts fabricated according to Examples 4-6,
while those in FIG. 5B are of catalysts fabricated according to
Examples 7-13. In FIGS. 5A, 5B the heavy line represents a scan of
substrate only, while light lines represent scans of catalysts at
varying catalytic cation loading densities. Scans of catalysts are
offset from the scans of substrate, for clarity. Catalysts
fabricated using protected catalytic cations, as shown in FIG. 5B
show large increases in evolved water at increased catalytic cation
loading density, particularly at around 300-400.degree. C.
Catalysts fabricated using unprotected catalytic cations, as shown
in FIG. 5A show much smaller increases in evolved oxygen even at
high loading densities.
[0104] FIGS. 6A, 6B, show TPR peak deconvolution and assignment for
catalysts fabricated according to Examples 9 and 12, respectively.
The light solid line shows the acquired data for the catalyst while
the dotted line shows the background TPR scan of ceria substrate.
The dash-dotted line(s) indicate new, deconvoluted peaks,
representing the difference between TPR of catalyst and substrate.
The heavy solid line shows the computed spectrum of background plus
400.degree. C. peak, and its close agreement with the acquired
spectrum. FIG. 6A shows only a single deconvoluted peak centered at
about 400.degree. C., while FIG. 6B has a much larger deconvoluted
peak centered at about 400.degree. C. and a second deconvoluted
peak centered at about 325.degree. C. Because of its lower
temperature and presence only in catalysts fabricated at high
loading densities, the reduction event centered at about
325.degree. C. is assigned to reduction of ferric catalytic cations
in three-dimensional aggregates. The reduction event centered at
about 400.degree. C. is assigned to reduction of ferric catalytic
cations in two-dimensional sheets.
[0105] FIG. 7 is a graph of total evolved oxygen as a function of
Fe.sup.3+ catalytic cation loading density, for catalysts
fabricated using protected cations (solid line) and unprotected
catalytic cations (dotted line). The slope of the solid line, 0.23
moles of H.sub.2O per mole of Fe.sup.3+ catalytic cation,
corresponds to reduction of 45% of the catalytic cations in the
catalyst fabricated using protected catalytic cations over the
temperature range of the TPR (because two Fe.sup.3+ cations must be
reduced for each H.sub.2O molecule produced). By contrast, the
slope of the dotted line, 0.11 moles of H.sub.2O per mole of
Fe.sup.3+ catalytic cation, corresponds to reduction of only 22% of
the catalytic cations in the catalyst fabricated using unprotected
catalytic cations over the temperature range of the TPR.
[0106] Taken together, these results suggest that use of protected
catalytic cations in the method results in a catalyst with
catalytic cations which are bound in lower proportion as isolated
ions and in higher proportion in two-dimensional sheets. The
results further suggest that the increase in population of
two-dimensional sheets results in a greater fraction of catalytic
cations which are available for reduction within the tested
temperature range.
Examples 41-44
Fe.sup.3+ Fe.sup.2+ Redox Cycling
[0107] Multiple cycles of TPR (i.e. redox cycles) were performed on
catalysts fabricated according to Examples 4-13. This enabled an
assessment of the redox cycling capability of catalytic cations,
and in particular of catalytic cations bound to substrate surfaces
in different geometries.
[0108] Samples were analyzed as in the TPR experiments (Examples
31-35), with the following additional steps. After the initial TPR
scan, samples were cooled to near ambient temperature in 100 sccm
helium, and then oxidized, cooled, and gradually reduced a second
time, as in the initial TPR experiment.
[0109] FIGS. 8A, 8B show TPR scans of first and second reductions,
at two catalytic cation loading densities, for catalysts fabricated
using unprotected catalytic cations and protected catalytic
cations, respectively. As seen in FIG. 8A, the second reduction was
the same as the first reduction for catalysts fabricated using
unprotected catalytic cations, except that the reduction feature
centered at about 400.degree. C. was shifted to about 380.degree.
C. As seen in FIG. 8B, the second reduction was essentially the
same as the first reduction for catalysts fabricated using
protected catalytic cations when the catalytic cations were
incorporated at low loading densities. At high loading densities
however, the reduction event centered at about 325.degree. C.,
assigned to reduction of catalytic cations incorporated in
three-dimensional crystallites, disappeared completely. This
suggests that catalytic cations incorporated in three-dimensional
crystallites are relatively unable to undergo redox cycles, as can
be the case for bulk oxides.
[0110] Also observable in FIG. 8B, the 400/380.degree. C. reduction
event decreased by about 42% on the second TPR cycle for the
catalyst fabricated according to Example 12. FIGS. 9A, 9B
illustrate the fractional loss of catalytic cations capable of
undergoing a second reduction in catalysts fabricated using
protected catalytic cations at high loading densities. Note that
the catalysts of FIG. 9B, fabricated using protected catalytic
cations at low-to-moderate loading density according to Examples 7,
10, and 11 possess a catalytic cation fraction of about 45% which
is capable undergoing multiple oxidation/reduction cycles under
conditions where reduction temperatures do not exceed 550.degree.
C.
Examples 45-49
X-Ray Spectroscopy
[0111] X-ray Absorption Spectroscopy experiments were performed on
catalysts fabricated according to Examples 4, 9, 10, and 12 in
order to evaluate oxidation state as a function of temperature and
surface geometry of catalytic cations at varying loading densities
and for unprotected or protected catalytic cations.
[0112] X-ray absorption near edge structure (XANES) spectroscopy
experiments were carried out on the Dupont-Northwestern
Collaborative Access Team (DND-CAT) bending magnet D Beamline at
Sector 5 of the Advanced Photon Source, Argonne National
Laboratory. Incident and transmitted intensity were measured with
Canberra ionization chambers. Beam energies were calibrated against
Fe metal foil measured in transmittance and its K-edge was set to
7112 eV. FeO and Fe.sub.2O.sub.3 standards were brushed on Kapton
tape and their spectra were measured also in transmittance.
[0113] Supported Fe materials were pressed into 50 mg pellets 2.5
cm in diameter. Single pellets were mounted on a stainless steel
heated cell designed for transversal gas flow and sealed with a Be
back window and Kapton front window. The void between the two
windows was filled with quartz wool before placing the pellet to
ensure it remained against the front Kapton sheet during
experiments. Due to the low Fe content of the pellets, spectra were
measured as fluorescence intensity using a four-channel SII
Vortex-ME4 detector. Samples were mounted at incident angle
.theta.=45 X with respect to the beam and detector, which were
perpendicular to one another. Before starting the temperature
program, spectra of the pellets loaded in the sample holder were
compared to those of the corresponding powder materials brushed on
Kapton tape and measure also as fluorescence intensity.
[0114] Following this, transversal flow of 50 sccm 3.5% H.sub.2 and
balance He started. The heated flow cell was held at ambient
temperature, 150.degree. C., 250.degree. C., 340.degree. C., and
430.degree. C. for two hours each while spectra were collected.
Temperatures are measured values inside the cell. Between holds,
the temperature was ramped at .about.10.degree. C.min.sup.-1.
[0115] FIG. 10 shows representative XANES spectra of four different
catalysts, each analyzed across a temperature range of
20-430.degree. C. Black dashed lines show reference spectra of
ferrous and ferric oxide. All materials tested showed pre-edge
features more intense of the bulk oxides, characteristic of
undercoordinated iron species. The pre-edge features diminished in
intensity for the catalysts fabricated using protected catalytic
cations, suggesting decreased catalytic cation dispersion on
substrate surfaces. The pre-edge feature is also more intense for
the catalyst fabricated using unprotected catalytic cations (FIG.
10D) as compared to the catalyst fabricated using protected
catalytic cation at a similar loading density (FIG. 10B).
[0116] FIG. 11 shows the fraction of Fe species existing as ferric
or ferrous cation at various temperatures for a catalyst fabricated
using unprotected catalytic cations and for a catalyst fabricated
using protected catalytic cations. The fraction of Fe species
existing as ferric or ferrous cation under each condition was
determined by linear interpolation of the spectra in FIG. 10
between the ferrous and ferric oxide reference spectra. The results
are in nearly exact agreement with fractional reduction determined
by TPR. In particular, the results show a greater fraction of
reducible ferric species in the catalyst fabricated using protected
catalytic cations.
Examples 50-55
Catalytic Reduction of NO by H.sub.2
[0117] Kinetic measurements of nitric oxide reduction reactions,
with hydrogen gas as reducing agent and in the presence of
catalysts fabricated according to Examples 4-30 were made in order
to evaluate the catalytic efficiency of catalysts as a function of
catalytic cation identity, catalytic cation protected or
unprotected nature, and promoter cation identity and loading
density.
[0118] NO reduction by H.sub.2 was carried out at atmospheric
pressure in a quartz U-tube reactor (OD 1/4'', ID 1/8'') charged
with 25 mg of catalyst diluted in 0.50 g of non-porous quartz sand
previously calcined to 600.degree. C. and packed in down flow
between beds of quartz wool. The U-tube reactor was lowered into an
upright tube furnace (ID 2.5'') and a thermocouple set against the
top of the catalyst bed was used to control and record bed
temperature. The thermocouple and quartz diluent showed no
catalytic activity in control experiments. The reactor inlet was
connected to a gas manifold and the outlet to a Pfeiffer Thermostar
mass spectrometer. The reactor was then fed 50 sccm 3.3% NO, 3.3%
H.sub.2 3.3% Ar, 90% He, heated at 10.degree. C.min.sup.-1 to
150.degree. C., 250.degree. C., 350.degree. C., 450.degree. C., and
550.degree. C. and held at each temperature for 1 h. Complete
disappearance of signals corresponding to atmospheric N.sub.2,
O.sub.2, H.sub.2O, and CO.sub.2 as well as surface H.sub.2O was
observed within 20 minutes at 150.degree. C.
[0119] FIG. 12 shows the quantities of NH.sub.3 and N.sub.2 product
produced by a catalyst fabricated using protected catalytic cations
and by a catalyst fabricated using unprotected catalytic cations. A
reaction temperature of 450.degree. C. showed high specificity for
N.sub.2 production and showed large differences between catalysts
fabricated using protected catalytic cations as compared to
catalysts fabricated using unprotected cations, and was thus used
for future catalytic reduction experiments.
[0120] FIG. 13A shows the reaction rates of NO reduction for
catalysts fabricated according to examples 4-13. Also examined were
substrates loaded with 0, 0.25, 0.5, or 0.75 sodium promoter
cations per nm.sup.2, but no catalytic cations. The results agree
with the TPR experiments, showing that catalysts fabricated with
protected catalytic cations generally catalyze NO reduction at a
greater rate than do comparable catalysts fabricated with
unprotected catalytic cations. In addition, an activity plateau at
the highest loading densities for the catalyst fabricated with
protected catalytic cations agrees with the TPR results. Substrates
loaded with promoter cations but no catalytic cations all had the
same activity as did neat ceria, indicating there is no promoter
effect in the absence of catalytic cations. A star symbol on the
y-axis of FIG. 13A shows the corresponding reaction rate as
catalyzed by a reference catalyst consisting of 1% by weight
platinum (Pt) on alumina.
[0121] In FIG. 13B, turnover frequency is calculated based on the
fraction of ferric species capable of reduction, determined by TPR.
The turnover frequency of catalytic cites in a platinum based
catalyst is represented by a star. The results show that turnover
frequency is stable across loading densities, and is comparable to
that of a platinum based catalyst.
[0122] FIG. 14 shows reaction rate for various catalysts fabricated
using protected catalytic cations, wherein several alkali metals
are employed as promoter cations. The results indicate that the
alkali metals employed are comparably effective as promoter
cations. All of the alkali promoter cations increase the reaction
rate by about 30% when used in 1:1 proportion to the protected
catalytic cation, relative to the catalyst fabricated using
protected catalytic cations without the optional promoter cations.
This similarity suggests that the effect of promoter cations is
most likely not electronic, but may be steric. Without being bound
by any particular theory, one possible explanation for the uniform
effect of various promoter cations is that the promoter cations
occupy isolated sites on substrate surfaces, effectively blocking
such isolated sites and inhibiting unproductive occupancy by
catalytic cations. In such a model, catalytic cations would then
tend to coalesce on substrate surfaces into two-dimensional sheets
or, at high loading densities, three-dimensional crystallites.
[0123] FIG. 15 shows reaction rates as catalyzed by catalysts
fabricated according to Examples 14, 19-21. These catalysts were
fabricated using protected, ferric catalytic cations loaded at
.about.0.5 Fenm.sup.-2, with sodium promoter cation loading
densities varying from 0 to .about.0.75 Nanm.sup.-2. The results
indicate a linear dependence of reaction rate on promoter cation
loading density over the examined density range.
[0124] In FIG. 16, reaction rates for a set of similar catalysts
are shown. The catalysts of FIG. 16 were fabricated using
protected, cupric catalytic cations. As can be seen from comparison
of FIG. 16 to FIG. 12, the catalysts fabricated using protected,
cupric catalytic cations are somewhat superior to the catalysts
fabricated using protected, ferric catalytic cations. Without
wishing to be bound by any theory, this may be due to intrinsically
better performance of cupric cations or the use of higher amounts
of the promoter cations due to the precursor used. As seen in FIG.
17, the catalyst fabricated using protected, cupric catalytic
cations also show a rate plateau at high loading densities,
although the plateau occurs at lower densities compared to similar
catalysts fabricated using protected, ferric catalytic cations
(compare FIG. 17 to FIG. 13A).
Examples 56-58
Comparison to Cobalt
[0125] The results of NO reduction reaction rate measurements for
the three catalysts at three temperatures are shown in FIG. 18. As
shown in FIG. 18, the catalyst fabricated using protected catalytic
cations which are cobaltic catalyzed the reaction with a rate
similar to those of the catalysts fabricated using protected
catalytic cations which are ferric or cupric.
Examples 59-63
Carbon Monoxide and Propylene as Reducing Agents
[0126] Activity of NO reduction with carbon monoxide as reducing
agent was measured for catalysts prepared essentially according to
Examples 10, 11, and 13 but with minor loading density differences
(0.54, 0.89, and 1.32 wt %, respectively) and for unmodified
CeO.sub.2 substrate. Reaction and monitoring conditions were
similar to those of Examples 50-55 but under a gas flow of 1000 ppm
NO, 1000 ppm CO, balance He and at a flow rate of 100 sccm. GHSV
(gas hourly space velocity) was held constant at .about.30,000
h.sup.-1. Reactions were heated to 250.degree. C., 350.degree. C.,
450.degree. C., and 550.degree. C. and held at each temperature for
.about.1 h. A similar reaction was run using a catalyst prepared
according to Example 13 with 1200 ppm NO, 133 ppm C.sub.3H.sub.6,
balance He. The results of the carbon monoxide and propylene
experiments are listed in Table 2.
TABLE-US-00002 Example 250.degree. C. 350.degree. C. 450.degree. C.
550.degree. C. NO Reduction in CO Activity (.mu.mol
g.sub.cat.sup.-1 s.sup.-1) 10 0.45 1.77 3.13 5.32 11 0.52 1.99 3.41
5.38 13 0.58 2.40 4.14 6.86 substrate 0.03 0.43 0.99 2.79 NO
Reduction in C.sub.3H.sub.6 Activity (.mu.mol g.sub.cat.sup.-1
s.sup.-1) 13 0.00 0.00 0.81 3.09
[0127] The foregoing description relates to what are presently
considered to be the most practical embodiments. It is to be
understood, however, that the disclosure is not to be limited to
these embodiments but, on the contrary, is intended to cover
various modifications and equivalent arrangements included within
the spirit and scope of the appended claims, which scope is to be
accorded the broadest interpretation so as to encompass all such
modifications and equivalent structures as is permitted under the
law.
* * * * *